There are a variety of situations in which it is desired to expand a metal tube radially to form a tight, leak-proof joint with a surrounding structure. For example, large heat exchangers, particularly the type used in the steam generators of nuclear power plants, often employ a tube sheet, which is a metal plate as much as several feet in thickness through which hundreds of stainless steel or carbon steel tubes must pass. The tube sheet is initially fabricated with through holes of a suitable diameter in which the tubes are inserted. The tubes are then expanded against the sides of the holes by plastic deformation to seal the small crevices that would otherwise exist around the tubes. If these crevices were allowed to remain, they could collect corrosive agents, and would, therefore, decrease the predictable life-expectancy of the equipment.
The traditional technique for expanding tubes radially employs mechanical rolling. There are, however, a number of significant disadvantages associated with this technique. For example, mechanical rolling causes elongation of the tube with an accompanying decrease in the thickness of the tube walls. In addition, it is a time-consuming process that is difficult to employ in the case of longer tubes. The use of rolling also imposes a minimum dimension on the inside diameter of the tube in relation to the tube wall thickness, since it must be possible to insert rollers large enough to have suitable strength and rigidity.
For the above reasons, efforts have been made to develop techniques for expanding tubes by the application of hydraulic pressure. According to this newer technique, a mandrel is inserted in the tube and a pressurized working fluid is introduced through the mandrel into a small annular space between the mandrel and the tube. Fluid must be confined within the tube between two seals that surround the mandrel.
Since it is often necessary to swage large numbers of tubes as part of a single operation, ease of insertion of a swaging mandrel in the tubes is an important factor. Insertion, however, is usually difficult because the mandrel must carry seals that engage the inner surface of the tube tightly enough to prevent leakage of the working fluid. Tolerance variations between tubes result in greater difficulties when a seal large enough to work with the largest tubes must be inserted in the smallest tubes.
In some previously known hydraulic swaging mandrels, the sealing member that comes into direct contact with the working fluid has been an O-ring. In some instances, O-rings have been used in combination with a harder second sealing member.
An objective of the present invention is to provide a highly effective sealing member for use in a swaging apparatus that facilitates insertion of the mandrel. Another objective is to provide a mandrel in which such a seal cooperates advantageously with a centering mechanism.